Non-invasive imaging broadly encompasses techniques for generating images of the internal structures or regions of a subject, such as a person or object. One such imaging technique is known as X-ray computed tomography (CT). CT imaging systems measure the attenuation of X-ray beams that pass through the subject from numerous angles (often referred to as projection data). Based upon these measurements, a computer can process and reconstruct images of the portions of the subject responsible for the radiation attenuation.
Collimators are used to size and shape a beam of X-rays from a source (such as an X-ray tube) so that only the X-ray radiation traveling in desired directions can pass through to the subject being imaged. The collimator may be made from a material that substantially blocks X-rays. The collimator defines an aperture that allows the portion of the incoming X-ray beam moving in the desired direction to pass through the collimator to the subject and then to a detector of the imaging system which measures the attenuation of the X-ray beam through the subject.
The X-ray beam through the aperture of the collimator is projected onto the detector. For good image reconstruction, it is desirable that all or a given portion of a detector be uniformly covered by X-rays from the source. Some known CT imaging systems use detectors that are generally rectangular and curved with respect to a plane that is transverse to the X-ray beam. Use of a substantially planar collimator results in undesirable beam projection coverage of the detector because the beam projection through the flat aperture is distorted (e.g., a different shape) relative to the shape of the curved detector. This distortion reduces the efficiency of the imaging system because only the portions of the beam projection that overlap the curved detector are utilized for image reconstruction. The areas of the beam projection outside of the detector unnecessarily expose the subject to excessive radiation that is not utilized for image reconstruction.
To size and shape the X-ray beam to better match the curved detector than the planar collimators, some known CT imaging systems have collimators with curved surfaces that define the aperture. Although the curved collimators can size and shape the imaging beam to match the detector, the curved collimators generally are not able to block scatter X-ray radiation from escape from the collimator towards the subject. This scatter X-ray radiation is partially-attenuated X-rays that are reflected and/or refracted by various internal features of the collimator. The escaping scatter radiation may be absorbed by the subject, thereby unnecessarily increasing the dosage to the subject, without being received by the detector and used for image reconstruction.
Thus, presently known collimators result in undesired overdoses of X-ray exposure.